1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and more particularly to a tandem color image forming apparatus including a plurality of photoconductive elements arranged side by side and each being rotatably supported at opposite end portions in the main scanning direction.
2. Description of the Background Art
A tandem color image forming apparatus, for example, includes a plurality of photoconductive drums or elements respectively assigned to a plurality of different colors, e.g., yellow, magenta, cyan and yellow and a plurality of optical writing devices respectively assigned to the drums. A laser beam issuing from each writing device and representative of a document image is focused on the surface of the drum associated therewith. A problem with the writing device is that when the surface of the drum on which the laser beam is focused is shifted in the direction of depth, the scanning position on the drum is also shifted in the main scanning direction. As a result, when images of different colors formed on the drums are superposed on each other, the colors are shifted from each other. The shift of the focusing position is ascribable to the oscillation and eccentricity of the drum in the radial direction.
In light of the above, Japanese Patent Laid-Open Publication Nos. 6-250474 and 2001-249523, for example, each teach that to make the shifts of a plurality of color images superposed on each other inconspicuous, vertical lines at each ends of an image in the direction perpendicular to the direction of sheet conveyance are matched to each other as to the phase of waving. However, even this kind of scheme is not fully satisfactory.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Publication No. 6-90561 (=Japanese Patent Laid-Open Publication No. 62-178988) and Japanese Patent Laid-Open Publication No. 7-140753.